1. Field of the Invention
This invention relates to mobile radio systems and more specifically to transmitting, receiving and demodulating digital information in a radio system.
2. Description of Related Art
In digital mobile radio systems, base units communicate with a plurality of mobile radio units. A transmitter of one of the communicating units transmits symbols embedded in a continuous radio-frequency (RF) signal allowing a receiver to decode the RF signal into digital information.
Channel impairments such as RF interferences, radio signal "fades", thermal receiver noise, and other RF distortion may cause errors in the detected message.
Often in ground-based communications, transmitted radio signals are reflected from physical objects and arrive at the receiver's antenna via paths of different lengths. Movement of either transmitter or receiver causes these paths to change constantly. Signals received from paths of different lengths arrive at slightly different times and, therefore, are sometimes out of phase. This is known as multi-path propagation. During multi-path propagation, the signal received by the mobile receiver exhibits time-varying signal strength as the mobile receiver moves through a standing wave field pattern. Alternatively, a mobile transmitter can cause a fixed receiver to receive a time varying field, which is also dependent upon the relative locations of the antennae.
Land mobile studies have found that the strength of signals received by antennae that are spatially separated by a distance of one quarter wavelength of the carrier frequency or more, are essentially uncorrelated. This property is the basis for the use of antenna diversity to improve the performance of land-based mobile radio systems.
In order to improve signal quality, a number of antennae have been connected to a receiver, and the signal from the antenna having the best quality is decoded. This is referred to as antenna diversity employing selection combining (SC). The receiver monitors the signal quality of each of the antennae, and periodically selects the signal of greatest quality, while ignoring the signals from other antennae.
"Linear Diversity Combining Techniques" by D. G. Brennan, pages 1075-1102, proceedings of the IRE, June 1959 discloses the use of maximal and ratio combining (MRC). Brennan discloses that combining of the signals with weighting factors proportional to their root mean square voltage results in a composite signal having the maximal signal to noise ratio. However, predetection maximal power ratio combining methods require signal envelope information and co-phasing of the signals from all of the antennae. Accurate co-phasing of fading signals from different antennae is a difficult task. Also, estimation of the signal envelope requires increased receiver complexity.
Currently there is a need for a simplified radio system which does not require signal co-phasing and signal envelope information.